Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Statement Of Cash Flows [Abstract]
Net loss	$ (11,410)	$ (11,779)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Depreciation and depletion	17,530	17,451
Amortization	3,130	2,827
Reserve for doubtful accounts	(447)	1,878
Asset impairments		76
Gain on sale of fixed assets	(714)	(112)
Deferred income taxes and taxes payable	119	(16,139)
Refundable income taxes	1,945	2,948
Stock compensation expense	2,416	1,653
Change in operating assets and liabilities:
Accounts receivable	(15,956)	(9,608)
Inventories	(9,038)	1,103
Prepaid expenses and other assets	(1,078)	3,286
Accounts payable	12,981	1,980
Accrued expenses and deferred revenue	26,489	(4,150)
Net cash provided by (used in) operating activities	25,967	(8,586)
Cash flows from investing activities
Proceeds from sale of fixed assets	957	588
Capital expenditures and stripping costs	(7,025)	(13,744)
Other investing activities	(758)
Net cash used in investing activities	(6,826)	(13,156)
Cash flows from financing activities
Payments on long-term debt	(2,170)	(3,128)
Payments on capital leases and other long-term debt	(817)	(1,724)
Proceeds from option exercises	486	101
Tax effect of stock options exercised, forfeited, or expired		(738)
Transactions with non-controlling interest	(1)	(535)
Net cash used in financing activities	(2,502)	(6,024)
Change in cash and cash equivalents related to assets classified as held-for-sale		34
Foreign currency adjustment	(44)	118
Increase (decrease) in cash and cash equivalents	16,595	(27,614)
Cash and cash equivalents:
Beginning of period	194,069	171,486
End of period	$ 210,664	$ 143,872